I. Field of the Invention
This disclosure relates generally to titanium (Ti) alloys. In particular, alpha-beta Ti alloys having an improved combination of mechanical properties achieved with a relatively low-cost composition are described as well as methods of manufacturing the Ti alloys.
II. Background of the Related Art
Ti alloys have found widespread use in applications requiring high strength-to-weight ratios, good corrosion resistance and retention of these properties at elevated temperatures. Despite these advantages, the higher raw material and processing costs of Ti alloys compared to steel and other alloys have severely limited their use to applications where the need for improved efficiency and performance outweigh their comparatively higher cost. Some typical applications which have benefited from the incorporation of Ti alloys in various capacities include, but are not limited to, aeroengine discs, casings, fan and compressor blades; airframe components; orthopedic components; armor plate and various industrial/engineering applications.
A conventional Ti-base alloy which has been successfully used in a variety of applications is Ti-6Al-4V, which is also known as Ti 6-4. As the name suggests, this Ti alloy generally contains 6 wt. % aluminum (Al) and 4 wt. % vanadium (V). Ti 6-4 also typically includes up to 0.30 wt. % iron (Fe) and up to 0.30 wt. % oxygen (O). Ti 6-4 has become established as the “workhorse” titanium alloy where strength/weight ratio at moderate temperatures is a key parameter for material selection. Ti 6-4 has a balance of properties which is suitable for a wide variety of static and dynamic structural applications, it can be reliably processed to give consistent properties, and it is comparatively economical.
Recently, the design of new aircraft engines has been driven by airline demands for reduced atmospheric emissions and noise, reduced fuel costs, and reduced maintenance and spare part costs. Competition between engine builders has caused them to respond by designing engines with higher bypass ratios, higher pressures in the compressor, and higher temperatures in the turbine. These enhanced mechanical properties require an alloy that has a higher strength than Ti 6-4, but the same density and near equivalent ductility.
Other alloys, such as TIMETAL® 550 (Ti-4.0Al-4.0Mo-2.0Sn-0.5Si) and VT 8 (Ti-6.0Al-3.2Mo-0.4Fe-0.3Si-0.15O), gain approximately 100 MPa of strength compared to Ti 6-4 from the inclusion of silicon in the alloy. However, these alloys have a higher density and a higher production cost, compared to Ti 6-4, because they use molybdenum as the main beta stabilizing element, as opposed to vanadium. The cost premium arises not only from the greater cost of molybdenum relative to vanadium, but also because the use of Ti 6-4 turnings and machining chip as a raw material is precluded in those alloys.
Therefore, there is a need in the industry to provide a cost-effective alloy that has a higher strength, finer grain size, and a particularly improved Low Cycle Fatigue Life with a comparable density when compared to Ti 6-4.